1. Field of the Invention
The present invention generally relates to optical scanners, image forming apparatuses, and buffer control methods for optical scanners, and more particularly to an optical scanner having a LED (Light Emitting Diode) array, and to a technique used in an image forming apparatus and a buffer control method in order to control storage and transfer of image data when correcting a sub scan magnification of the LED array.
2. Description of the Related Art
In the image forming apparatus employing the electrophotography technique, such as a printer and a copying apparatus, an electrostatic latent image is optically formed on a surface of an image bearing member, such as a photoconductive drum, by a laser beam or the like. An image is formed by adhering toner on the electrostatic latent image.
When an eccentricity is generated in the photoconductive drum of the image forming apparatus, a peripheral velocity of the surface of the photoconductive drum varies periodically. As a result, an error may be generated in the line interval or pitch in a paper transport direction (or sub scan direction) of the electrostatic latent image that is formed on the surface of the photoconductive drum. When the error in the line interval or pitch is generated, an image quality of the image that is finally formed on paper or the like may deteriorate due to an inconsistency in tone, a partial expansion or contraction of the image, and the like.
Methods have been proposed to prevent such a deterioration of the image quality, by detecting an amount of error in the line interval or pitch. For example, when optically forming the electrostatic latent image, an amount of light of the laser beam or the like emitted in the sub scan direction may be corrected depending on the detected amount of error in the line interval or pitch.
When a LED array in which a plurality of LEDs are arranged is used in order to optically form the electrostatic latent image, the cause of the image quality deterioration may be prevented by correcting a light emission period of the LED array.
One example of the method to prevent the image quality deterioration is proposed in a Japanese Laid-Open Patent Publication No. 2007-144731. According to this proposed method, an image forming apparatus includes an exposure unit, a moving unit, and a light emission control unit. The exposure unit includes a plurality of light emitting parts arranged along a first direction, and the moving unit moves the exposure unit and an image bearing member relative to each other in a second direction perpendicular to the first direction. The light emission control unit forms an image on the image bearing member by periodically emitting the light from the light emitting parts of the exposure unit, depending on image data representing the image to be formed on the image bearing member. In addition, the light emission control unit varies a light emission period of the light emitting parts during formation of the image, in order to correct a periodic variation in at least one of a tone and a magnification along the second direction within the image formed on the image bearing member.
According to the image forming apparatus employing this proposed method, the periodic variation in at least one of the tone and the magnification along the sub scan direction within the image may be corrected, because the light emission period of the light emitting parts is varied in order to correct the periodic variation in at least one of the tone and the magnification along the second direction perpendicular to the first direction within the image formed on the image bearing member.
The image data are temporarily stored in a buffer when emitting the light from the LED array. However, the buffer needs to have a large capacity when correcting the sub scan magnification by varying the light emission period of the LED array, because the amount of image data to be stored in the buffer becomes large.
On the other hand, in order to drive the LED array at a high speed, the buffer is desirably provided within an ASIC (Application Specific Integrated Circuit) that processes the image data. However, when the buffer is provided within the ASIC, there is a limit to increasing the capacity of the buffer. In addition, increasing the capacity of the buffer inevitably increases the cost.
Hence, according to the above described image forming apparatus employing the proposed method, an amount of the sub scan magnification correctable by the LED array becomes limited when the buffer does not have a sufficiently large capacity. When the amount of the sub scan magnification correctable by the LED array is limited and a satisfactory correction is impossible, the deterioration in the image quality of the image finally formed on paper or the like may not be sufficiently suppressed.